Field of the Invention
The present invention relates to the field of optical fibers. In particular the present invention relates to a method of micro- and nano-structuring fiber ends to enhance transmission of light over a wide range of wavelengths and increase laser damage threshold.
Description of the Prior Art
Typical optical fibers that transmit light in the 0.4 to 1.6 micron range are fabricated from silica glass. Silica glasses are low-index materials having a refractive index of about 1.4 to about 1.5, which is near the 1.0 refractive index of air. Consequently, light passes through the glass-air interface without significant transmission loss, frequently referred to as Fresnel loss. Typically silica optical fibers have a transmission loss in the near infrared of about 4% loss per interface.
For the mid infrared regions and beyond (e.g. beyond 1.6 μm), optical fibers are typically composed of high index materials, such as chalcogenide glasses. Chalcogenide glasses have high refractive indices of about 2.4 to about 2.8; the light consequently experiences high losses of about 17% to 22% loss per interface when it enters and exits the fiber to and from air, respectively.
A number of different techniques have been developed to reduce transmission loss at optical fiber ends. For example, the change in refractive index at the ends of optical fibers can be reduced by applying an anti-reflective coating on the fiber tip. These coatings take advantage of the interference phenomenon which occurs in thin films and therefore can be designed to enhance the light transmission within a defined wavelength band where constructive interference takes place so as to reduce the reflection on the fiber end. While these coatings are fairly robust in the case of silica-based glasses, they have limitations for infrared materials. In the case of chalcogenide glasses, which cannot be subjected to very high temperatures, the coatings have poor adhesion to the chalcogenide glass and are sensitive to humidity. Additionally, these coatings damage easily under intense laser radiation. Consequently, there is a need for reducing surface reflection losses using a more robust approach and for enabling higher laser power transmission through increased laser damage threshold.
Transmission losses can also be reduced by incorporating a plurality of sub-wavelength surface (SWS) relief structures on the fiber end so as to induce the refractive index to gradually vary from the refractive index value of surrounding medium (air) to the refractive index value of the window material. These SWS relief structures are generally a collection of objects, such as graded cones, pillars or, similarly, depressions that generate strong diffraction or interference effects due to certain periodic or quasi-random distribution of said objects. The distances between the objects and the dimensions of the objects themselves are typically smaller than the wavelength of light with which they are designed to interact.
The SWS approach has been successfully used on a variety of bulk substrates from glasses to ceramics to optical crystals and polymers. Photolitography followed by plasma etching (for periodic patterns) or simply plasma etching (for quasi-random patterns) have been the methods of choice for SWS structuring of bulk substrates.
Little has been done, to date, to create SWS directly into the ends of optical fibers and without the help of extrinsic materials (such as depositing a coating of nanoparticles or depositing a layer of a soft material which is further nanostructured as needed). Neuberger et al. (U.S. Pat. No. 6,208,781) teaches how a silica fiber end can be structured using a patterned molybdenum stamp. Sanghera et al. (Opt. Expr. 18, 26760) teaches how a chalcogenide fiber can be structured using a patterned nickel or silicon stamp. However, both consider only periodic patterns for the stamp and, hence, are demonstrating the case of fiber end structuring with a periodic pattern. Additionally, no actual attempt has been done, to date, for developing and disclosing a system capable of industrial-like processes for fiber end structuring.